1. Field of the invention
The invention concerns a method and a device for testing the function capacity of an NO oxidation catalyst.
2. Description of Prior Art
Internal combustion engines of motor vehicles operated with air surplus are fitted with catalytically functioning post-treatment systems to reduce pollution, which systems comprise NOx storage catalysts, SCR catalysts and also particulate filters, in order to be able to comply with legally prescribed exhaust emission limits. The common feature of all these systems is that nitrogen dioxide (NO2) constitutes an important component of the reactions proceeding in the post-treatment systems. This nitrogen dioxide is formed by usually platinum-containing NO oxidation catalysts (NO=nitrogen monoxide) and the nitrous oxides (NOx) contained in the exhaust gas, from the nitrogen monoxide emitted by the combustion engine:2 NO+O22NO2   Equation 1
In real operation however the sulphurisation of these NO oxidation catalysts by the sulphur contained in the fuel and/or the engine oil constitutes a considerable problem, Due to combustion, this sulphur leads to the formation of sulphur dioxide (SO2) which is oxidised on the NO oxidation catalyst into SO3 according to the equations below:S+O2→SO2  Equation 22 SO2+O2→SO3  Equation 3
Here it has been found that the quantity of SO3 formed and the quantity of NO2 formed are directly correlated, so that an NO oxidation catalyst which desirably forms large quantities of NO2 at the same time generates undesirably large quantities of SO3. This SO3 with the metal-containing catalyst washcoat forms sulphates, or with water forms sulphuric acid, which are physiosorbed on the surface of the catalyst. Both lead to coverage of the active centre of the catalyst and hence a fall in its activity, which in turn reduces the activity of downstream exhaust gas post-treatment components such as SCR catalysts or particulate filters.
For this reason it is necessary to monitor the state of an NO oxidation catalyst or its oxidation capacity.
For three-way catalysts, a method is known from DE 10 2004 009 615 B4 in which the oxygen (O2) storage capacity is determined using periodic fluctuations between lean and rich engine operation and observation of the catalyst reaction using a downstream lambda sensor, and structured such that in the case of a damped amplitude of residual O2 fluctuation, the catalyst still has an O2 storage capacity, while in the other case it is no longer able to absorb O2. This method cannot be applied to NO oxidation catalysts for two reasons: firstly these catalysts have no pronounced O2 storage capacity, and secondly diesel engines which normally run lean or direct-injection petrol engines cannot easily be run rich. Here, both the soot emissions and the thermal load on the engines would rise undesirably and to an extreme degree.
In so-called diesel oxidation catalysts which serve to oxidise unburnt hydrocarbons and carbon monoxide, a second method is known from EP 1 373 693 B2. Here periodically the hydrocarbon oxidation is raised, usually by late post-injection of fuel into the combustion chamber, and the exothermy of the oxidation of these hydrocarbons on the diesel oxidation catalyst is determined using thermo-elements. The temperature rise detected is compared with an expected value determined from the added quantity of hydrocarbons. If the measured and expected values deviate too greatly from each other, damage to the diesel oxidation catalyst can be concluded. The disadvantage of this method is that large quantities of hydrocarbons must be added, since otherwise because of the large thermal mass and the resulting low temperature rise, no system reaction can be observed. This leads to a clear deterioration in the efficiency of the internal combustion engine and hence to a rise in fuel consumption. A further disadvantage lies in the high thermal load of the catalyst which, for the NO oxidation catalysts already described above, can even lead to their damage. In particular in internal combustion engines fitted in vehicles, because of the widely varying ambient conditions and the resulting fluctuating temperature losses of the exhaust gas system, the problem can also arise that temperature detection can be subject to a high measurement inaccuracy.
EP 1 936 140 A1 already discloses a method for monitoring an exhaust gas post-treatment system of an internal combustion engine in which a first lambda sensor to detect the air ratio is arranged in the exhaust gas flow upstream before the exhaust gas post-treatment system, and in which a second lambda sensor to detect the air ratio is arranged further downstream of the exhaust gas post-treatment system. To monitor the function capacity of the exhaust gas post-treatment system, the internal combustion engine is brought into an operating mode in which the exhaust gases extracted from the cylinders contain such a high concentration of unburnt hydrocarbons that the first sensor works defectively, such that this first sensor indicates a higher air ratio in comparison with the air ratio actually present in the exhaust gas, wherein the malfunction of the exhaust gas post-treatment system is assumed if the two air ratios are substantially the same.